This invention relates to the field of asymmetric hollow fibers, particularly those that comprise an ultrathin selective layer for air and gas separations.
In creating a gas or liquid separation membrane, it is desirable to have both a high rate of permeation or throughput and a high separation factor or selectivity. This combination of characteristics permits the effective separation of a relatively large volume of fluid per unit time. The phase inversion process is one of the most important means to prepare asymmetric membranes for air separation. The resultant membranes have a dense skin layer that is integrally bonded in series with a thick porous substructure. The skin and the substructure are composed of the same material.
The skin layer, which contains the effective separating layer, is one of the key elements in determining the membrane permeability and selectivity. To have a high-performance air-separation membrane, this skin layer has to be as thin as possible and must contain a minimum of defects.
U.S. Pat. No. 4,871,494 issued to Kesting, et al. describes a process for forming asymmetric gas separation membranes having graded density skins. This process comprises dissolving a glassy polymer in a Lewis acid: base complex solvent system wherein the Hildebrand parameters of the solvent species and the polymer are within 1.5. The spinning dope prepared by this method comprises a polymer and a solvent mixture, but the solvent mixture further contains two components.
U.S. Pat. No. 4,902,422 issued to Pinnau and Koros describes solution formulations to prepare defect-free ultrahigh flux flat asymmetric membranes using a forced convective drying process. In order to form the skin layer, this process requires solvents which either have low boiling points or high vapor pressures in the solution system. The cast solution prepared by this method has multiple components, and at least consists of a polymer, a low boiling point solvent and a high boiling point solvent.